Solid reactor with an antistatic coating for carrying out reactions in a gaseous phase

ABSTRACT

The inner wall of a solids reactor for carrying out reactions in the gas phase is coated with an antistatic layer having a thickness of 0.1-800 mum and consisting essentially of a poly-alpha-olefin and a nonvolatile antistatic agent.

The present invention relates to solids reactors for carrying outreactions in the gas phase and to plant components for handlingfluidized solids, whose inner wall is coated with a thin antistaticlayer consisting essentially of a poly-α-olefin and a nonvolatileantistatic agent. It further relates to a process for coating suchreactors and plant components, the use of such reactors for thepolymerization and copolymerization of a α-olefins and to a process forthe polymerization of α-olefins using a coated reactor.

In the polymerization of a α-olefins in the gas phase, deposits arefrequently formed on the walls of the reactor. It is known that thisdeposit formation is at least partly attributable to electrostaticcharging, as disclosed, for example, by WO 86/07065. Owing toelectrostatic forces, catalyst and polymer particles adhere to the wallof the reactor and finally bake together under the action of the heat ofpolymerization liberated to form solid deposits. These deposits can falloff and lead to blocking of the product discharge system. They thuscause problems in the continuous operation of such polymerizationplants, increase the need for cleaning and can make more frequentstopping of the plant necessary. In addition, in the gas-phasefluidized-bed process, the fluidization behavior of the bed is adverselyaffected.

Problems with electrostatic charging are also known in the handling ofpoly-α-olefins. Thus, for example, electrostatic charging can easilyoccur when conveying poly-α-olefins in pneumatic conveying systems orwhen filling and emptying silos and this can lead to wall deposits andblockages. Electrostatic charging can also be the cause of dustexplosions.

Electrostatic charging is influenced in a complex way by numerous systemparameters of the gas-phase polymerization process, for example by theparticle size distribution of the polymer and of the catalyst, thechemical composition of the catalyst, the internal reactor temperature,the reaction pressure and the composition of the circulating gas.

To solve this problem, it has been proposed that the polymerization becarried out in the presence of various antistatic agents. Thus, forexample, U.S. Pat. No. 4,855,370 discloses the use of water asantistatic agent, U.S. Pat. No. 5,026,795 discloses mixtures ofpolysulfone copolymers/polyamines and a sulfonic acid, EP-A 364 759discloses Kerostat® (mixture of chromium stearylanthranilate, calciummedialanate and di-t-butylphenol), EP-A 584 574 discloses mixtures ofalcohol phosphate salts and quaternary ammonium salts, EP-A 653 441discloses the use of naphthoquinone dimer compounds and EP-A 636 636discloses metal salts of anthranilic acid. The use of Stadis® 450 (EP-A803 514, polyamine/polysulfone) or of particularly suitable amines (EP-A811 638) has been proposed specifically for α-olefin polymerizationusing metallocene catalysts. Furthermore, U.S. Pat. No. 4,803,251 hasproposed measuring the electrostatic potential in the reactor during thepolymerization and, depending on the presence of excess positive ornegative charge, using exactly the correct amount of either methanol ormethyl isobutyl ketone to neutralize the respective charge. However,this process is complicated in terms of measurement and regulation.

Although the problems in respect of deposits on the reactor wall can belargely solved by means of the processes described, they all have thedisadvantage that the antistatic agent or its solvent, e.g. propanol,introduced into the reaction space can reduce the activity of thecatalysts used. Only low catalyst productivities are therefore achieved.Metallocene catalysts in particular are extremely sensitive to polarcomponents in the antistatic agent. Furthermore, polar components canmodify the catalysts and thus change the product properties.

It has therefore also been proposed (WO 86/7065) that the reactor wallbe treated with a chromocene compound in order to reduce theelectrostatic charging of the reactor. However, this treatment takesfrom a number of hours to a number of days. In addition, chromocene iscomplicated and expensive to prepare and is very sensitive to impuritiesand therefore difficult to handle. Furthermore, the antistatic actiondoes not last long.

It has also been proposed (RD 23803 (1984)) that the inner wall of thereactor be sprayed with a composition comprising an aromatic polyimide,Teflon and pigments such as chromium oxides or iron oxides and themixture be crosslinked. The coating formed in this way has a thicknessof 1-3 mm. However, a thick, Teflon-containing layer of this type isvery expensive. In addition, the spraying and crosslinking of such alayer can only be carried out with the reactor open, so that the coatingprocedure or any necessary repairs to the coating can only be carriedout after the production plant has been shut down.

Coating with comparatively soft polymers is generally problematicalsince it has to be feared that they might be abraded by hardconstituents of the fluidized bed, e.g. catalyst particles.

It is an object of the present invention to find an antistatic coatingfor solids reactors for carrying out reactions in the gas phase which ischeap, simple and quick to apply and repair, which has good durabilityand, in particular, is not abraded under the conditions for thepolymerization of α-olefins in the gas phase.

We have found that this object is achieved by a coating comprisingpolyolefins and nonvolatile antistatic agents.

The present invention accordingly provides reactors for carrying outreactions in the gas phase and plant components for handling fluidizedsolids whose inner wall is coated with a thin antistatic coating havinga thickness of 0.1-800 μm and consisting essentially of a poly-α-olefinand a nonvolatile antistatic agent. We have also found a process forcoating such reactors and plant components, the use of such reactors forthe polymerization and copolymerizaton of α-olefins and a process forthe polymerization of α-olefins using coated reactors.

The reactors of the present invention can be all types of reactors whichcan be used for carrying out reactions in the gas phase. For thepurposes of the present invention, the term reaction is not restrictedto chemical reactions but also includes other chemical engineeringoperations which can be carried out in the gas phase, for example dryingor classification in the gas phase. The reactors of the presentinvention are preferably used for reactions of organic solids, inparticular for the polymerization of α-olefins in the gas phase.Possible reactor types are, in particular, stirred autoclaves,fluidized-bed reactors, stirred fluidized-bed reactors, fluidized-bedreactors with a circulating fluidized bed or flow tubes.

Furthermore, reactors for other gas-phase operations used in chemicalengineering can be treated according to the present invention, forexample reactors which can be used for drying fluidized solids, inparticular organic solids, e.g. fluidized-bed driers or spray driers.

The plant components treated according to the present invention can beany components of chemical plants in which solids, in particular organicsolids and very particularly preferably poly-α-olefins, are fluidized,for example pipes and other components of pneumatic conveying systems orsilos.

The inner wall is coated with a layer having an antistatic action.Preference is given to coating the entire reaction space, but it is alsopossible to coat only those parts of the wall of the reactor on whichdeposits are preferentially formed during the course of the reaction.The thickness of the antistatic layer on the reactor wall is from 0.1 to800 μm, in particular from 1 to 100 μm, particularly preferably from 5to 10 μm.

The antistatic layer consists essentially, i.e. to an extent of at least90% by weight based on the sum of all constituents, of a poly-α-olefinand a nonvolatile antistatic agent.

Preferred poly-α-olefins are polymers of α-olefins having from 2 to 8carbon atoms; particular preference is given to polyethylene andpolypropylene. The invention also encompasses copolymers of variousα-olefins. Preferred comonomers for polyethylene are propene, 1-butene,1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene and 1-octene and alsomixtures of these α-olefins. The poly-α-olefins can further compriseother comonomers, for example those containing polar groups, e.g.(meth)acrylic acid, (meth)acrylates or (meth)acrylamide. However, theproportion of such monomers preferably does not exceed 5 mol%, based onthe sum of all monomers. It is also possible to use mixtures of 2 ormore polymers.

The content of nonvolatile antistatic agent in the antistatic coating isfrom 0.1 to 50% by weight, based on the sum of all constituents. Thecontent of antistatic agent is preferably from 1 to 25% by weight andparticularly preferably from 5 to 15% by weight.

For the purposes of the present invention, the term nonvolatile coversall antistatic agents whose vapor pressure is sufficiently low for them,embedded in a matrix of poly-α-olefins, not to go into the gas phase, orat most to go into it in very small amounts, at temperatures of up to150° C. The term antistatic agent encompasses not only individualcompounds but also mixtures of various compounds having an antistaticaction.

Suitable nonvolatile antistatic agents are, for example, finely divided,porous carbon blacks, polyhydric alcohols and their ethers, for examplesorbitol, polyalcohols, polyalcohol ethers, polyvinyl alcohols,polyethylene glycols and their ethers with fatty alcohols, and anionicantistatic agents such as C₁₂-C₂₂-fatty acid soaps of alkali metals oralkaline earth metals, salts of alkyl sulfates of higher primary orsecondary alcohols having the formula ROSO₃M (M=alkali metal, alkalineearth metal) or (RR′)CHOSO₃M, salts of mixed esters of polyfunctionalalcohols with higher fatty acids and sulfuric acid, C₁₂-C₂₂-sulfonicacids or their salts of the formula RSO₃M, alkylarylsulfonic acids ortheir salts, e.g. dodecylbenzenesulfonic acid, phosphoric acidderivatives such as di(alkoxypolyethoxyethyl) phosphates of the formula[RO(CH₂CH₂O)_(n)] ₂POOM or phytic acid derivatives as disclosed, forexample, in EP-A 453116, cationic antistatic agents such as quaternaryammonium salts of the formula R₁R₂R₃R₄NX, where X is a halogen atom andR₁ to R₄ are alkyl radicals, preferably those having at least 8 carbonatoms. Also suitable are, for example, metal complexes such as thecyanophthalocyanines disclosed in WO 93/24562.

Particularly useful nonvolatile antistatic agents are nonvolatilenitrogen-containing compounds such as amines or amides or their salts,in particular oligomeric or polymeric amines and amides. Examples whichmay be mentioned are polyethoxyalkylamines or polyethoxyalkylamides ofthe formula R₁N[(R₂O)_(m)R] [(R₃O)_(n)H] or R₁CON[(R₂O)_(m)R][(R₃O)_(n)H] as described in DE-A 31 088 43, which are also constituentsof commercial antistatic agents (e.g. Atmer® 163 from ICI). It is alsopossible to use salt mixtures of calcium salts of medialanic acid andchromium salts of N-stearylanthranilic acid, as described in DE-A3543360, or mixtures of a metal salt of medialanic acid, a metal salt ofanthranilic acid and a polyamine, as described in EP-A 636 636.

Further particularly useful compounds are polyamines or polyaminecopolymers or mixtures of such compounds with further compounds, inparticular polymeric compounds. Apart from simple polyamines such aspolyvinylamine, suitable nonvolatile polyamines are advantageouslyobtained from the reaction of aliphatic primary monoamines such asn-octylamine or n-dodecylamine or N-alkyl-substituted aliphatic diaminessuch as N-n-hexadecylpropane-1,3-diamine and epichlorohydrin. Thesepolymers have hydroxyl groups in addition to amino groups. An overviewof such polyamine copolymers is given in U.S. Pat. No. 3,917,466.Polymers which are particularly suitable for use together withpolyamines or polyamine copolymers are polysulfone copolymers. Thepolysulfone copolymers are preferably largely unbranched and are made upof olefins and SO₂ units in a molar ratio of 1:1. An example is 1-decenepolysulfone. An overview of suitable polysulfone copolymers is alsogiven in U.S. Pat. No. 3,917,466.

Mixtures of polyamines and polysulfone copolymers are also constituentsof commercially available antistatic agents such as Stadis® 450 (DuPont)or Polyflo® 130 (Universal Oil Company).

The nonvolatile antistatic agent is preferably distributed homogeneouslyin the polymer matrix.

The choice of suitable polymers and suitable nonvolatile antistaticagents depends on the respective application and can be made largelyfreely. It is restricted only insofar as the softening point of theantistatic coating has to lie above the temperature at which the desiredreaction or process step is to be carried out. The polymer chosen forthe antistatic coating is preferably one of the same type as that whichis to be produced or handled, i.e., for example, apolyethylene-containing coating for the production of polyethylene.

The coating applied according to the present invention may furthercomprise additional additives and auxiliaries in small amounts, e.g.ones for improving the adhesion of the layer to the reactor wall. Theamount of such constituents should, however, never exceed 10% by weight,based on the sum of all constituents.

The coating provided according to the present invention can be appliedin a simple way by treating the wall of the reactor or the plantcomponent with a poly-α-olefin at the softening point and a nonvolatileantistatic agent.

The process can be carried out, for example, by introducing a sufficientamount of the desired poly-α-olefin and the desired antistatic agentinto the reactor. The mixture is distributed uniformly in the reactor,e.g. by stirring by means of a built-in stirrer or by fluidization usingan inert gas stream, preferably a stream of nitrogen, and the reactor isheated to the softening point of the polymer. It is usual to select atemperature which is from about 5 to 15° C., preferably from 8 to 10°C., below the melting point of the polymer. The antistatic agent isadded either in finely divided form as a solid or advantageouslydissolved in suitable solvents. The addition of the antistatic agent canbe carried out before, during or immediately after heating. The solventand any further volatile constituents present in the antistatic agentevaporate so that only the nonvolatile components of the antistaticagent remain. The duration of the treatment depends on the conditionsselected, but is normally less than 4 hours. The treatment of thereactor can be carried out at atmospheric pressure or at asuperatmospheric pressure of preferably not more than 80 bar. Thethickness of the antistatic layer formed can be set as a function of thesoftening temperature of the poly-α-olefin used, the temperature and thecoating time.

Even though the method indicated is preferred, the present invention isnot restricted thereto, but also encompasses application of the coatingto the inner wall of the reactor in any other way, e.g. by dissolvingthe polymer and the nonvolatile antistatic agent in a suitable solvent,spraying-on the solution and evaporating the solvent.

The reactor of the present invention is very well suited to thepolymerization and copolymerization of α-olefins in the gas phase.Ethylene and propylene and in particular ethylene can be homopolymerizedor copolymerized particularly readily. Possible comonomers are, inparticular, α-olefins having from 3 to 8 carbon atoms, speciallypropene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-hepteneand 1-octene and also mixtures of these α-olefins.

The polymerization is carried out at from 30 to 150° C., preferably from80 to 120° C. The pressure is from 5 to 80 bar, preferably from 20 to 60bar.

The polymerization can be carried out by various gas-phase processesusing the abovementioned antistatically coated reactors, i.e. forexample in gas-phase fluidized beds or in stirred gas phases. Suchgas-phase processes are known per se and are described, for example, inUllmann's Encyclopedia of Industrial Chemistry, Vol. A 21, 4th edition1992, p. 511 ff.

The process of the present invention can be carried out using variouscatalysts as are customary for the polymerization of α-olefins. Examplesof suitable catalysts are the supported chromium catalysts also known asPhillips catalysts. Further catalysts which can be used in the processof the present invention are supported Ziegler catalysts orZiegler-Natta catalysts. Customary supported catalysts of theabovementioned types are described, for example, in Ullmann'sEncyclopedia of Industrial Chemistry, Vol. A 21, 4th edition 1992, p.501 ff. It is also possible to use a supported metallocene catalyst,e.g. as disclosed in EP-A 803 514, as catalyst or constituent of thecatalyst mixture. Other organometallic, polymerization-active complexescan also be used.

The coating applied according to the present invention is effective inreducing the electrostatic charging of the reactors of the presentinvention during the course of the polymerization. Instead of a chargeof up to several thousand volts without a coating, the charge on areactor coated according to the present invention fluctuates only aroundzero. Wall deposits can thereby be avoided virtually completely and theyields of the reaction rise significantly. This effect surprisinglyoccurs even at a coating thickness of a few μm.

The coating applied according to the present invention has the furtheradvantage that it can be applied in a short time. The existing,customary inlet and outlet points present on the plant can be used forcarrying out the coating process, so that it is not necessary to open orenter the reactor in order to carry out the coating process. The coatingagents are advantageously distributed in the reactor by means of thestirrers installed in the reactor or the fluidization facilities andheating is advantageously carried out using the existing heatingfacilities. As a result of the low coating thickness, only a smallamount of material is consumed in the coating process. Coating of thereactor is thus cheap.

Particularly surprisingly, the antistatic coatings applied according tothe present invention have a very high mechanical stability anddurability under operating conditions. To remove the antistatic layer,the inner wall of the reactor has to be polished thoroughly using anabrasive material. The antistatic action lasts for a number of months.

The reactors of the present invention therefore make a very usefulcontribution to more economical production of poly-α-olefins.

The following examples illustrate the invention without restricting itsscope.

The measured properties described were determined in the following way:

Density In accordance with ISO 1183 Staudinger index In accordance withISO 1628 Electric potential measured using a commercial electric fieldmeasuring instrument; the measuring electrode was located in the middleof the reactor 2 cm above the bottom, the tip of the probe was insulatedfrom the reactor wall; the reactor was grounded. Thickness of themeasured by means of ultrasound in antistatic layer running-timemeasurements

EXAMPLE 1

Coating of the reactor

A 1 l autoclave was charged under a nitrogen atmosphere with 300 g ofpolyethylene of the grade Lupolen® 6021 D (melting point: 131° C.) and,while stirring at 120° C. under 30 bar of nitrogen, 40 ml of a 25%strength solution of the antistatic agent Stadis® 450 (from DuPont,mixture of dodecylbenzenesulfonic acid, a copolymer of epichlorohydrinand N-alkyl-1,3-diaminopropane and an alternating polysulfone copolymerof 1-decene and SO₂, solvent: toluene, isopropanol and 1-decene) inhexane were added. After 3 hours, the autoclave was emptied. A 5 μmthick layer of polyethylene and the antistatic agent had formed on thereactor wall.

EXAMPLE 2

The procedure of Example 1 was repeated using polyethylene of the gradeLupolen® 4261 A (melting point: 125° C.) and a coating temperature of115° C. The thickness of the antistatic layer formed was 7 μm.

EXAMPLE 3

The procedure of Example 1 was repeated using polyethylene of the gradeLupolen® 3621 D (melting point: 120° C.) and a coating temperature of110° C. The thickness of the antistatic layer formed was 10 μm.

EXAMPLE 4

Preparation of a Phillips catalyst having a Cr content of 1%

185 g of silca gel (SG 332, Grace) was suspended in 400 ml of a solutionof Cr(NO₃)₃*9 H₂O (3.56% by weight) in methanol. After 30 minutes,methanol was taken off and the catalyst precursor obtained was activatedwith air at 650° C.

EXAMPLES 5 AND 6

Polymerization experiments at 110° C. and an ethylene pressure of 40 barusing the chromium(VI) catalyst described in Example 4 were carried outin the autoclave which had been coated as described in Example 1. Dataregarding the polymerization conditions, the product properties and theelectrostatic behavior during the polymerization are summarized in Table1.

EXAMPLES 7 AND 8

Polymerization experiments at 110° C. and an ethylene pressure of 40 barusing the chromium catalyst described in Example 4 were carried out inthe autoclave which had been coated as described in Example 2. Dataregarding the polymerization conditions, the product properties and theelectrostatic behavior during the polymerization are summarized in Table1.

EXAMPLES 9 AND 10

Polymerization experiments at 110° C. and an ethylene pressure of 40 barusing the chromium catalyst described in Example 4 were carried out inthe autoclave which had been coated as described in Example 3. Dataregarding the polymerization conditions, the product properties and theelectrostatic behavior during the polymerization are summarized in Table1.

COMPARATIVE EXAMPLES 11 AND 12

Polymerization experiments at 100° C. and an ethylene pressure of 40 barwere carried out as in Examples 5 and 6, except that an uncoated reactorwas used in place of a coated reactor. Data regarding the polymerizationconditions, the product properties and the electrostatic behavior duringthe polymerization are summarized in Table 1.

COMPARATIVE EXAMPLE 13

200 g of a polyethylene powder which had a density of 0.9465 g/cm³ and ahigh load melt flow rate of 6.5 g/10 min and had been dried at 80° C.for 10 hours were introduced under nitrogen into an uncoated 1 lautoclave. The autoclave was pressurized with 30 bar of nitrogen. Whilststirring, the autoclave was heated to 100° C. over a period of 30minutes. At 400 rpm, a charge of −4500 V/m was measured. After stirringfor 1 hour, the reactor was cooled to room temperature and opened. Thereactor wall was covered with a 1 cm thick layer of polyethylene powder.

EXAMPLE 14

Comparative Example 13 was repeated except that a reactor which had beencoated as described in Example 3 was used. The electric fieldmeasurement indicated no charging (±200 V/m). The reactor wall was freeof polyethylene particles.

The examples show that electrostatic charging in the polymerizationexperiments in the coated reactor is considerably less than in theuncoated reactor. In the experiments in the uncoated reactor, largeamounts of wall deposits were formed, so that the yield of theexperiments was significantly reduced.

TABLE 1 Polymerization experiments in an antistatically coated gas-phaseautoclave Density Electric of the PE Staudinger field PolymerizationPolyethylene Productivity obtained index η intensity No. Coating time[min] yield [g] [g PE/g cat.] [g/cm³] [dm³/g] [V/m] Example 5 Stadis ®450/ 90 245 2900 0.9512 5.83 +/− 190* Lupolen ® 6021 D Example 6Stadis ® 450/ 90 255 3000 0.9503 5.28 +/− 180* Lupolen ® 6021 D Example7 Stadis ® 450/ 90 270 2500 0.9490 6.14 +/− 180* Lupolen ® 4261 AExample 8 Stadis ® 450/ 90 230 2800 0.9515 5.19 +/− 175* Lupolen ® 4261A Example 9 Stadis ® 450/ 70 225 2600 0.9495 5.53 +/− 175* Lupolen ®3621 D Example 10 Stadis ® 450/ 70 210 2600 0.9507 6.37 +/− 185*Lupolen ® 3621 D Comparative none 90 135 2300 0.9489 5.30 −2500 Example11 Comparative none 90 205 2700 0.9492 5.55 −2300 Example 12*Measurement fluctuates around zero from −175 (−180 V) to +175 (+180 V)

We claim:
 1. A solids reactor for carrying out reactions in the gasphase whose inner wall of the reaction space is coated with anantistatic layer having a thickness of 0.1-800 μm and consistingessentially of a poly-α-olefin and a nonvolatile antistatic agent.
 2. Aplant component in which solids are fluidized, whose inner wall iscoated with an antistatic layer having a thickness of 0.1-800 μm andconsisting essentially of a poly-α-olefin and a nonvolatile antistaticagent.
 3. The solids reactor or plant component of claim 1, wherein theantistatic layer contains 0.01-50% by weight of a nonvolatile antistaticagent.
 4. The solids reactor or plant component of claim 1, wherein thenonvolatile antistatic agent is a nitrogen-containing, oligomeric orpolymeric compound.
 5. The solids reactor of claim 1, which is a stirredautoclave, a fluidized-bed reactor, a stirred fluidized-bed reactor, afluidized bed reactor with a circulating fluidized bed or a tubereactor.
 6. A process for coating a solids reactor for carrying outreactions in the gas phase and for coating plant components in whichsolids are fluidized, which comprises treating their inner wall with apoly-α-olefin at the softening point and a nonvolatile antistatic agent.7. A process for the polymerization or copolymerization of α-olefins inthe gas phase at from 30 to 150° C. and a pressure of from 5 to 80 barusing a supported catalyst, wherein the polymerization is carried out inthe coated solids reactor of claim 1.